1. Field of the Invention
The present invention relates to a multiblade fan typified by a sirocco fan, and more particularly to a technology for reducing noise generated by a multiblade fan. The present invention also pertains to an electronic apparatus having a low-noise multiblade fan.
2. Description of the Related Art
Various technologies have been developed in recent years for reducing noise generated by home electric appliances. Demands for lower noise are not limited to home electronic appliances, but also apply to electronic devices. In particular, in the field of personal computers, not only personal computers per se, but also peripheral devices such as printers and display units, are required to reduce noise.
Electronic devices have components such as magnetic disk drives, CPUs, power supplies, and light sources which generate heat during operation. These heat-generating parts need to be cooled in order to maintain their performance and reliability. There are generally two types of designs for cooling heat-generating parts, i.e., air cooling designs and liquid cooling designs. The air cooling designs are grouped into the category of natural air cooling systems and the category of forced air cooling systems. According to the forced air cooling systems, a cooling fan forcibly applies air to a heat-generating component or air is forcibly discharged from an electronic device. If a higher cooling effect is to be achieved, then the operating sound of the cooling fan tends to be perceived as noise. Generally, electric devices having larger heat-generating parts are liable to generate larger sounds from cooling fans.
One electronic device that has larger heat-generating parts is a projector. In terms of cooling mechanisms, conventional projectors are classified into projectors of the type which cool heat-generating parts with an axial fan and projectors of the type which cool heat-generating parts with a multiblade fan (sirocco fan). A projector of the former type has an air intake fan for introducing external air and delivering the introduced air through an air filter to a liquid crystal display unit, and an air exhaust fan for discharging hot air in the projector out of the projector. Some projectors also have an auxiliary fan in addition to the air intake fan and the air exhaust fan.
A projector of the latter type has a multiblade fan (sirocco fan) and a cooling duct. Air around a heat-generating component is drawn through the cooling duct into the multiblade fan, and discharged out of the projector through a exhaust port disposed in the front portion of the projector (see Japanese laid-open patent publication No. 11-354963).
As described above, a projector cooling mechanism employs an axial fan or a multiblade fan. In general, multiblade fans typified by a sirocco fan are widely used to cool devices having large internal resistance because the multiblade fans are capable of developing higher static pressure than the axial fans.
Attempts have been made to reduce noise produced by the cooling mechanism for projectors. For example, Japanese laid-open patent publication No. 10-197953 discloses a technique to reduce noise with a grid member having a plurality of rectangular or circular openings which is disposed in an upstream position in an air passage that is located near to an air exhaust fan. Japanese laid-open patent publication No. 2000-35614 reveals another arrangement for reducing noise with a net-like flow rectifier which is disposed at the downstream end of a heat sink with respect to the air flow through the heat sink. Japanese laid-open patent publication No. 2001-142147 shows still another scheme for lowering noise with a sound silencer box having an air passage chamber through which air discharged from a cooling mechanism passes.
Efforts have also been made to reduce noise produced by the cooling multiblade fan itself. For example, Japanese laid-open patent publication No, 10-141294 discloses a technique for reducing noise with a barrier wall which is disposed within the blades of impeller and which suppress the vortex flow. Multiblade fans typified by a sirocco fan produce more noise, called nz-noise, at frequencies offensive to the ear than axial fans because the pressure of air varies when the blades of the impeller pass in the vicinity of the tongue of the fan case, generating nz-noise whose frequency is based on the frequency of the air pressure variations. The frequency f (Hz) of nz-noise produced by a sirocco fan is expressed by f=n×z/60 where n represents the rotational speed (rpm) of the sirocco fan and z the number of blades of the impeller. When a general small-type sirocco fan having 30 blades rotates at a rotational speed of 5000 rpm, the frequency of produced nz-noise is expressed as f=5000×30/60=2500 (Hz). Since this nz-noise frequency falls into a frequency range from 1 kHz to 4 kHz that is most offensive to the ear, the nz-noise is perceived as annoying noise even if its level is low.
It has been proposed to provide multiblade fans with a fin for suppressing the above offensive noise. However, it is difficult to incorporate the proposed fin into small-size multiblade fans since it would be necessary to increase the number of blades or to increase the rotational speed of the fan in order to shift the frequency of nz-noise into a less offensive frequency range while at the same time causing the fan to produce high static pressure. Stated otherwise, technical difficulties will be encountered in producing dies capable of forming small-size multiblade fans having more blades than up to the present. In addition, an increase in the rotational speed will cause a reduction in the durability of fans.